Soil Stabilization Using Fly Ash and Quarry Dust
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Abstract
About This Research Topic
Beneath a lot of Nigerian roads sits a soil problem that never quite goes away: weak, high-plasticity lateritic clay that swells, shrinks, and simply can't carry traffic load the way a subgrade needs to. The usual fixes, hauling in fresh granular fill or dosing the soil with lime or cement, work, but they're expensive, and on a country's worth of rural highway projects, that cost adds up fast. Meanwhile, two industrial waste products, fly ash from power plants and quarry dust from granite crushing, pile up in stockpiles and landfills with barely any productive use.
This article draws on a study that put those two waste materials to work, blending fly ash and quarry dust in equal parts and testing how well the mix improves a genuinely problematic lateritic subgrade soil, one that started out well below the minimum strength Nigerian highway specifications require. For readers curious how a geotechnical study like this is designed and run, our sample research projects library includes comparable materials and pavement engineering studies worth reviewing as models.
The results speak directly to a real cost-and-sustainability question facing Nigerian highway agencies: is there a cheaper, locally available alternative to imported stabilisers that still gets the job done? The sections below cover the background to the problem, what the study found, and what it means for subgrade improvement practice going forward.
Main Abstract
How well a flexible pavement performs, and how long it lasts, ultimately comes down to the bearing capacity and volumetric stability of the subgrade soil beneath it. A large share of the subgrade soils found along Nigerian highway alignments are problematic, high-plasticity lateritic clays with low bearing capacity, high swell potential, and poor performance under repeated traffic loading and changing moisture. This study investigated whether fly ash and quarry dust, two industrial waste by-products from thermal power generation and granite quarrying respectively, blended in equal proportion, could improve the engineering properties of one such problematic soil, classified as A-7-6 under the AASHTO system, with a natural liquid limit of 52 percent, a plasticity index of 28 percent, and a soaked California Bearing Ratio of just 2.1 percent, far below the 10 percent minimum Nigerian highway design practice typically requires.
The natural soil was stabilised with the fly ash-quarry dust blend at 10, 20, 30, and 40 percent by dry weight, and the study measured the resulting index properties, compaction characteristics, soaked and unsoaked CBR, unconfined compressive strength, and free swell index at each stabiliser content.
Plasticity index fell steadily as stabiliser content rose, from 28 percent for the untreated soil down to 11 percent at 40 percent stabiliser content, while free swell index dropped from 58 percent to 15 percent over the same range, both signs of meaningfully improved volumetric stability. Soaked CBR climbed from 2.1 percent for the natural soil to a peak of 13.6 percent at 30 percent stabiliser content, before dipping slightly to 12.8 percent at 40 percent, marking 30 percent as the sweet spot where the soil comfortably cleared the 10 percent minimum subgrade CBR requirement set out in the Nigerian General Specification for Roads and Bridges. Unconfined compressive strength followed the same pattern, peaking at 650 kPa at 30 percent stabiliser content, up from just 145 kPa for the untreated soil.
Statistical analysis backed up these findings: a second-order polynomial regression showed a strong relationship between stabiliser content and soaked CBR, and one-way ANOVA confirmed the differences across stabiliser content levels were highly significant. At the optimum 30 percent stabiliser content, the soil's AASHTO classification jumped from A-7-6, a poor subgrade material, to A-2-4, a good one, confirming a genuine, practical improvement in subgrade quality rather than just a marginal statistical shift.
The study concludes that a fly ash-quarry dust blend at 30 percent by weight offers an effective, low-cost, and environmentally beneficial way to upgrade problematic lateritic subgrade soils to meet Nigerian highway specification requirements, while also giving two industrial waste materials a genuinely productive use instead of sending them to landfill. It recommends that highway agencies consider fly ash-quarry dust stabilisation as a real alternative to imported or costlier conventional stabilisers such as lime or Portland cement wherever similar problematic soils turn up on Nigerian road projects.
Chapter One Preview
Background to the Study
The subgrade layer, the natural or prepared soil that a pavement structure is built on top of, fundamentally determines how well a flexible highway pavement performs over its service life, since it's the ultimate foundation supporting every layer above it and the traffic loads they carry. A significant share of the subgrade soils found along Nigerian highway alignments, particularly across the country's extensive tropical lateritic soil zones, come with real engineering problems: high plasticity, weak bearing capacity, and significant volumetric instability as moisture conditions shift with the seasons. Left unimproved, these characteristics contribute directly to premature pavement distress, rutting, cracking, and localised failure among them.
Where subgrade soils fall short of minimum bearing capacity requirements, usually measured through the California Bearing Ratio test, highway engineers have traditionally turned to one of two options: removing the problematic soil entirely and replacing it with imported granular fill, which can be prohibitively expensive and logistically difficult on remote project sites, or chemically stabilising the soil in place using additives like lime or Portland cement, which work well technically but come with a real material cost and, in cement's case, a significant embodied carbon footprint. These trade-offs have driven growing research interest in industrial waste by-products as cheaper, more locally available, and more environmentally sound alternatives for improving problematic subgrade soils.
Fly ash, a fine particulate by-product of pulverised coal combustion in thermal power generation, and quarry dust, a fine by-product from crushing and screening aggregate at granite quarries, are two industrial waste materials generated in real volume in Nigeria, both mostly disposed of through landfilling or open stockpiling today, an environmental liability that's also, in a sense, a wasted resource. The US EPA's work on coal combustion residual reuse documents exactly this kind of beneficial reuse pathway for fly ash internationally. Fly ash carries recognised pozzolanic properties that let it contribute to soil stabilisation through a cementitious reaction with soil moisture and, where present, calcium-bearing compounds, while quarry dust, though largely chemically inert, can improve soil gradation, reduce plasticity, and enhance compaction through its well-graded, granular particle size distribution.
This study investigated the combined use of fly ash and quarry dust, blended in equal proportion, as a stabilising additive for a problematic lateritic subgrade soil representative of what's found along Nigerian highway alignments, with the aim of pinning down an optimum stabiliser content capable of bringing the soil up to the minimum subgrade specification set out by the Federal Ministry of Works, the agency responsible for Nigeria's federal road standards.
Statement of the Problem
Nigerian highway construction projects frequently run into problematic, high-plasticity subgrade soils that fail to meet the minimum bearing capacity, or CBR, requirements set out in prevailing highway design and specification documents, forcing a choice between costly soil replacement or chemical stabilisation with additives like lime or Portland cement, both of which carry a material cost burden that can meaningfully affect overall project cost, especially for rural or remote highway work. At the same time, substantial quantities of fly ash and quarry dust are generated as industrial waste in Nigeria with limited productive use, creating both an environmental disposal burden and a missed opportunity to use a locally available construction resource.
While fly ash and quarry dust's individual and combined stabilisation potential has been studied in various international settings, there's been limited empirical data specifically characterising how a fly ash-quarry dust blend performs as a stabilising additive for problematic lateritic subgrade soils representative of the Nigerian highway context, using materials actually sourced within Nigeria. This study addresses that gap through a systematic experimental investigation of fly ash-quarry dust blend stabilisation applied to a representative problematic lateritic subgrade soil, generating locally grounded evidence to inform how it might be applied in Nigerian highway subgrade improvement practice.
Aim and Objectives of the Study
The aim of this study is to investigate the use of fly ash and quarry dust, blended in equal proportion, as a stabilising additive for improving the engineering properties of a problematic lateritic subgrade soil for road subgrade applications.
The specific objectives of the study are to:
● Determine the physical, index, and chemical properties of the natural subgrade soil, fly ash, and quarry dust used in this study.
● Stabilise the natural soil with a fly ash-quarry dust blend (1:1 ratio) at 10%, 20%, 30% and 40% by dry weight of soil.
● Determine the Atterberg limits (liquid limit, plastic limit, and plasticity index) of the soil at each stabiliser content.
● Determine the compaction characteristics (maximum dry density and optimum moisture content) of the soil at each stabiliser content.
● Determine the soaked and unsoaked California Bearing Ratio (CBR) of the soil at each stabiliser content.
● Determine the unconfined compressive strength (UCS) and free swell index of the soil at each stabiliser content.
● Assess the improvement in AASHTO soil classification achieved at each stabiliser content.
● Establish the statistical relationship between stabiliser content and soaked CBR, and determine the optimum stabiliser content that satisfies minimum Nigerian subgrade specification requirements.
Research Questions
This study seeks to answer the following research questions:
● What are the physical, index, and chemical properties of the natural subgrade soil, fly ash, and quarry dust used in this study?
● How do the Atterberg limits of the soil vary with increasing fly ash-quarry dust stabiliser content?
● How do the compaction characteristics of the soil vary with increasing stabiliser content?
● What is the effect of stabiliser content on the soaked and unsoaked CBR of the soil?
● What is the effect of stabiliser content on the unconfined compressive strength and free swell index of the soil?
● How does the AASHTO soil classification of the soil change with increasing stabiliser content?
● Is there a statistically significant relationship between stabiliser content and soaked CBR, and what is the optimum stabiliser content for satisfying Nigerian subgrade specification requirements?
Justification/Significance of the Study
This study is significant in several respects. It generates locally grounded empirical evidence on using fly ash and quarry dust, two industrial waste materials generated in real volume in Nigeria, as a subgrade stabilising additive, addressing a gap given how much of the existing combined fly ash-quarry dust stabilisation literature originates internationally rather than from Nigerian materials and conditions.
The findings offer direct practical value to highway agencies, geotechnical engineers, and road construction contractors looking for a cost-effective, locally accessible alternative to conventional stabilisers like lime or Portland cement for improving problematic subgrade soils on Nigerian highway projects. Students and early-career engineers working on similar soil stabilisation, materials testing, or regression-based geotechnical analysis can get direct feedback on methodology through our research coaching service, and browse related technical studies in our civil engineering project archive.
The study also supports environmental sustainability goals by giving two industrial waste materials, which would otherwise need landfill disposal, a genuinely productive, value-adding use, reducing their environmental burden while solving a real geotechnical engineering challenge. It further offers a reference framework for future research into industrial waste-based soil stabilisation, and gives Nigerian highway specification and standards bodies empirical evidence to help shape appropriate guidelines for using fly ash-quarry dust blends in subgrade improvement work.
Scope of the Study
This study is limited to the experimental investigation of a fly ash-quarry dust blend, mixed in equal proportion by weight, as a stabilising additive for a single representative problematic lateritic subgrade soil, at stabiliser contents of 10%, 20%, 30%, and 40% by dry weight of soil. It covers Atterberg limits, compaction characteristics (standard Proctor), soaked and unsoaked CBR, unconfined compressive strength, free swell index, and AASHTO soil classification.
The study does not extend to the durability performance of the stabilised soil under repeated wetting-drying or freeze-thaw cycling, nor does it examine alternative fly ash-quarry dust blend ratios beyond the 1:1 ratio adopted, both recommended for further research.
Operational Definition of Terms
Subgrade: The natural or prepared soil layer upon which a pavement structure (sub-base, base, and surface courses) is constructed, providing the ultimate foundation support for the pavement system.
California Bearing Ratio (CBR): A penetration test-based index of the bearing capacity of a soil, expressed as a percentage relative to a standard crushed stone material, widely used in pavement design to characterise subgrade and other pavement layer materials.
Atterberg Limits: A set of soil index tests, comprising the liquid limit, plastic limit, and plasticity index, used to characterise the plasticity and consistency behaviour of fine-grained soils at varying moisture content.
Fly Ash: A fine particulate by-product of pulverised coal combustion in thermal power generation, possessing pozzolanic properties useful for soil and concrete stabilisation applications.
Quarry Dust: A fine particulate by-product generated during the crushing and screening of aggregate at quarrying operations, typically consisting of finely divided rock particles.
Free Swell Index (FSI): A measure of the volumetric expansion potential of a soil when submerged in water, used to characterise the swelling potential of expansive clay soils.
AASHTO Soil Classification System: A soil classification system developed by the American Association of State Highway and Transportation Officials, widely used in highway engineering to classify subgrade soils according to their suitability for pavement support, ranging from A-1 (best) to A-7 (poorest).
Conclusion
Two waste materials that would otherwise end up in a landfill turned out to be enough to take a genuinely poor subgrade soil and bring it up to standard. At 30 percent stabiliser content, the fly ash-quarry dust blend didn't just nudge the numbers, it pushed soaked CBR from 2.1 percent to 13.6 percent and shifted the soil's AASHTO classification from poor to good. For highway agencies weighing the cost of imported stabilisers against a locally available, waste-derived alternative, this study offers a concrete, tested answer rather than a theoretical one. Readers interested in related pavement and geotechnical engineering research can browse more civil engineering project topics for further reading.
Frequently Asked Questions (FAQs)
What is the optimum fly ash-quarry dust content for stabilising subgrade soil?
This study identified 30 percent stabiliser content, by dry weight of soil, as the optimum, the point at which soaked CBR peaked at 13.6 percent and clearly exceeded the 10 percent minimum required by Nigerian subgrade specifications.
How much does fly ash-quarry dust stabilisation improve CBR?
Soaked CBR rose from just 2.1 percent for the untreated soil to 13.6 percent at the optimum 30 percent stabiliser content, a more than sixfold improvement that took the soil from well below to comfortably above the minimum subgrade requirement.
Can industrial waste materials replace lime or cement for soil stabilisation?
This study suggests they can, at least for this soil type: a fly ash-quarry dust blend at 30 percent achieved the required subgrade improvement at what is likely a lower material cost than lime or Portland cement, while also giving two waste materials a productive use.
What is the AASHTO soil classification, and how did it change in this study?
The AASHTO system classifies subgrade soils from A-1 (best) to A-7 (poorest) based on suitability for pavement support. In this study, the natural soil started at A-7-6, a poor subgrade classification, and improved to A-2-4, a good subgrade classification, at the optimum 30 percent stabiliser content.
How does fly ash-quarry dust stabilisation affect soil plasticity?
Plasticity index dropped steadily as stabiliser content increased, from 28 percent for the untreated soil to 11 percent at 40 percent stabiliser content, indicating a substantial reduction in the soil's tendency to swell and shrink with moisture changes.
Why does soaked CBR decline slightly at 40 percent stabiliser content?
Soaked CBR peaked at 30 percent stabiliser content (13.6 percent) and dipped slightly to 12.8 percent at 40 percent, suggesting that beyond a certain point, additional stabiliser no longer contributes proportionally to strength gain, making 30 percent the more efficient, optimum content.
What is free swell index, and why does it matter for subgrade soils?
Free swell index measures how much a soil expands volumetrically when submerged in water, indicating its swelling potential. In this study, free swell index fell from 58 percent for the untreated soil to 15 percent at 40 percent stabiliser content, reflecting a major improvement in volumetric stability.
Is fly ash-quarry dust stabilisation environmentally beneficial?
Yes. Both fly ash and quarry dust are industrial waste by-products that are otherwise disposed of through landfilling or open stockpiling in Nigeria. Using them as a stabilising additive gives them a productive use while reducing the associated environmental disposal burden.
What soil type was used in this study?
The soil tested was a problematic lateritic subgrade soil classified as A-7-6 under the AASHTO system, with a natural liquid limit of 52 percent, a plasticity index of 28 percent, and a soaked CBR of only 2.1 percent.
Was the relationship between stabiliser content and CBR statistically significant?
Yes. A second-order polynomial regression model confirmed a strong relationship between stabiliser content and soaked CBR, and one-way ANOVA confirmed that the differences in soaked CBR across stabiliser content levels were statistically significant.
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